Sintering process



A. M. SCHWARZ SINTERING PROCESS Jan. 19, 1965 2 Sheets-Sheet 1 Filed Feb. 24, 1960 Invenfor Arihur Mfichwan;

fl-Horriega 33m, & Kw

United States Patent 3,166,403 SINTERING PROCESS Arthur M. Schwarz, Munster, Ind, assignor to Inland Steel Company, Chicago, IlL, a corporation of Delaware Filed Feb. 24, 1960, Ser. No. 10,780 7 Claims. or. 75-5 The present invention relates generally to the sintering of iron ore and more particularly to an improved method of producing a fluxed iron ore sinter'product having a low arsenic content from iron ore containing a relatively high percentage of arsenic.

There has been increasing recognition in the metallurgical art that the production of iron in a blast furnace can be significantly increased by charging thereto an iron ore sinter product having a balanced composition which contains all the components required for usein the blast furnace. More'particularly, the use of a 100% self-fluxing iron ore sinter in a blast furnace has been reported to increase the productivity of the blast furnace by as much as 74% (Blast Furnace and Steel Plant, vol. 47, No. 7, p. 697 (1959)). A blast furnace charged with the self-fluxing sinter has also been found to have a significantly lower rate of coke consumption and produces less carbon dioxide in the furnace, thereby resulting in a smoother and more efiicient operation. Accordingly, the production of a fluxed iron ore sinter is of increasing importance, and it is highly desirable to develop more economical methods of producing good quality fluxed iron ore sinters.

The conventional and most widely used method of sintering iron ore is by utilizing the Dwight-Lloyd type sintering apparatus which employs a continuously moving permeable sinter grate having associated therewith forced air boxes or wind boxes to create a forced air draft through the sinter bed. In general, an iron ore sinter partially fluxed (i.e., containing added basic matter) or a selffiuxing iron ore sinter (i.e., containing all basic reacting matter required to reactwith silica and other acidic content of ore) can be readily made on the Dwight-Lloyd type apparatus by adding limestone to the sinter feed mixture. However, when iron ores having a high arsenic content are subjected to the conventional process of producing fluxed iron ore sinter, the arsenic in the iron ore. cannot be completely or even largely removed during the sintering operation. as the limestone content in an iron ore sinter mixture increases from to the percentage of arsenic removed drops correspondingly from about 90% to 3%. And, while the removal of arsenic can be improved by increasing the coke breeze content of the iron ore sinter mixture to about 8%, it is generally considered necessary to limit the coke breeze content of an iron ore sinter mixture to about 6% to 6.5% in order to obtain satisfactory sinter quality. It is thus impractical with present iron ore sintering methods to prepare a satisfactory highly-fluxed or a self-fiuxing iron ore sinter from iron ore having a high arsenic content. As there is a considerable volume of high arsenic content iron ore available and otherwise suitable for sintering and using in a blast furnace, it is highly desirable to provide an improved method of removing arsenic from said iron ores and particularly, to providing a method of producing a highlyfluxed or self-fluxing iron ore sinter suitable for conventional use in a blast furnace from an iron ore having a high arsenic content.

It is, therefore, a primary object of the present invention to provide a more economical method of producing a fluxed iron ore sinter.

It is a further object of the present invention to provide Thus, it has been found that 3,166,403 Patented Jan. 19, 19 65 a method of producing an iron ore sinter having a low I arsenic content from an iron ore originally having a relatively high percentage of arsenic.

It is also an object of the present invention to provide an improved method of removing objectionable arsenic impurities from an iron ore.

It is still another object of the present invention to provide in a sintering process a more economical method of removing and collecting volatile arsenic impurities contained in an iron ore.

It is also an object of the present invention to provide an improved process utilizing a multi-la'y'er sinter bed for the production of a fluxed iron ore sinter while simultaneously producing a flux-free iron ore sinter.

, It is a further object of the present invention to provide a more economical method of producing continuously a fluxed iron ore sinter on a permeable moving sinter grate while simultaneously removing arsenic from an iron ore sinter mixture spaced from said fluxed sinter on the said moving grate.

A still further object of the present invention is to provide an improved method of separating a layer of. fluxed iron ore sinter from a layer of flux-free iron ore sinter produced simultaneously on a single moving sinter bed.

Other objects of the present invention will be apparent from the detailed description and claims to follow. 7

It has .beendiscovered that the foregoing and other objects of the present invention can be achieved in an economical manner without extensive modification of existing production equipment by providing a process and means whereby a sinter bed is formed of a plurality of distinct and separable layers of an iron ore sinter mixture disposed in a predetermined and at least partially superimposed relationship, each layer having a composition different from every other layer, and each layer of iron ore sinter mixture being sintered while being conveyed on the moving grate of a sintering apparatus, such as the Dwight-Lloyd type apparatus. Thus, in the practice of the present invention, at least two distinct horizontally disposed layers of sintering material in at least partial superimposed relationship are supported on a single con-, tinuously moving sinter grate, one of said layers'preferably being an acid or flux-free iron ore sinter mixture, and the other layer being a fluxed iron ore sinter mixture.

More particularly, in a preferred embodiment of the present invention, three contiguous layers of material are deposited on a moving grate of a Dwight-Lloyd type sintering apparatus to form a .multi-layer sinter bed with the spaced upper and lower layers each being formed of iron ore sinter feed mixture having a different chemical composition from the other and the intermediate layer therebetween being a relatively thin layer of a metallurgically useful material which does not fuse with itself or with the sinter feed mixtures at the sintering temperatures attained. When the multi-layer sinter bed is subjected to a sintering operation, a distinct metallurgically useful reaction takes place in the individual layers of the sinter bed and with the intermediate layer also providing a plane of cleavage permitting separation of the bed into two dinstinct iron ore sinter products.

One application of the present invention is the simultaneous production of both a flux-free or acid iron ore sinter product and a fiuxed iron ore sinter product. Thus, for. example, in some sintering installations it may be impossible to properly control the chemical composition of the final sinter product due to lack of adequate bedding facilities and proportioning devices in the sinter plant. In other instances it may be considered necessary or desirable to produce a sinter product providing a ratio of base to acid in excess of 1:3, the ratio usually considered as com- 3 prising a completely self-fiuxing sinter. In these instances the production of two types of sinter, one basic and the other acid, may be desirable as it makes possible the charging of a blast furnace by adding acid or basic sinter in the required proportion to provide a fully prepared 100% 'self-fluxing and uniform burden. The acid sinter product which is produced can also be recycled for use in the preparation of the sinter feed for the basic sinter layer. By utilizing up to about 50% acid sinter product in the basic sinter feed mixture, a substantially improved basic sinter product is produced due to the increased porosity and permeability thereof which, in addition, results in increasing the overall productivity of the sinter line.

A particularly useful application of the present invention, and an application for which the present invention is especially adapted, is the production of a high-grade fluxed sinter product suitable for use in a blast furnace from an iron ore containing a substantial proportion of a high 1 arsenic content iron ore. As heretofore indicated, it has been found that iron ores having a high arsenic content do not show any significant reduction in the arsenic content thereof after being subjected to a basic or fluxed sintering process wherein limestone is used as the fluxing agent. A conventional acid or flux-free sintering operation having a sufiicient fuel content, however, does effect a marked reduction in thearsenic content when the iron simultaneously producing a fluxed sinter product in a hori-. zontally disposed superimposed layer supported by the same continuously moving grate; said superimposed layer preferably containing a substantial proportion of the low arsenic, flux-free, sinter product. The present process is particularly adapted to continuously recycling all or a part of thelow arsenic, flux-free, acid sinter product produced in the non-fluxed sinter layer as a component of the 'iluxedsinter mixture layer, thus providing a highly efiicient means for continuous production of a low arsenic fluxed sinter product from a high arsenic iron ore.

While there are numerous ways. of providing for the convenient separation of the non-fluxed and the fluxed sinter layers, a very convenient and practical method of separating the said layers is to provide between the nonfluxed and the fluxed sinter feed layers a relatively thin horizontal layer of a metallurgically useful material which does not fuse'with either of the contiguous sinter feed layers or with itself at the sintering temperatures attained in the sinter bed. Thus,for example, a relatively thin layer of relatively finely divided limestone," when interposed between the upper and lower sinter feed layers, does not fuse with itself or fuse with the sinter layers and become an integral part of either of the said sinter layers, and

thus provides a plane of easy cleavage so that the acid or' flux-free sinter product can be readily separated from the fluxed sinter product at the. end of the sinter line when a separating blade, for example, contacts the layerof limestone and guides the fluxed sinter product into one storage area and the acid or flux free sinter product into a second storage area or onto a conveyor means for recycling. The limestone of the cleavage layer is also calcined during the sintering operation and thus is ideally suited for use in the preparation of the fluxed sinter feed mixture and for charging to a blast furnace along with the fluxed sintered product. If desired, coal, coke breeze, or other carbonaceous fuel, can be added to the limestone layer to pro- Vide for rapid ignition of the lower sinter layer, where an 4 1 unusually thick layer of limestone might tend to insulate the lower layer of sinter mixture from the burning upper layer of sinter and prevent spontaneous ignition of the said lower layer.

in the present invention, it is possible and generally desirable to regulate independently the maximum temperature attained in the individual sinter layers during sintering. Usually the. temperature is controlled by varying the amountof coke breeze or other fuel incorporated in the sinter feed mixture of the respective layers. As disclosed herein, however, the said temperature can also be controlled by controlling the temperature of the gas passing through the. sinter bed, such as employing preheated gas. The acid or flux-free sinter layer preferably is controlled so as to attain a maximum temperature during the sin-tering operation somewhat lower than the temperature of the upper layer, since it is desirable to have the acid'sinter product relatively frangible and the upper fluxed sinter product relatively resistant to crushing so that it will have better properties when added to the blast furnace. v

it has also been discovered that the'present iron ore sintering process employing the multiple layer sinter bed provides a simplified and more economical method of removing arsenic from the sinter bed by substantially reducing the area of the sinter bed from which the arsenic containing gases leave the sinter bed, thereby reducing very substantially the volume of gases which must be treated to remove'arsenic'therefrom before releasing to:

Thus, in one embodiment, the doublethe atmosphere. layer sintering process of the present invention has been found to reduce about in half the area of the sinter line from which arsenic containing gases are released. By having. the arsenic containing gases released from a limited portion of the sinter strand, it becomes unnecessary to pass the large volume of gases from the entire sinter strand through the arsenic removal apparatus and sub 'fluxed iron oresinter mixture will be ignited thereby and become fused with the acid sinter product, the fluxed sinter mixture is deposited directly on the upper surface of the acid sinter product. The upper surfaceof the layer of fiuxedsinter mixture is then ignitedand allowed to burn in the usual manner. In this embodiment of the invention the speed of longitudinal movement of the sinter grate and the depth of the fluxedsinter mixture are so correlated that there remains a thin layer of unburned fluxed sinter mixture when the sinter bed arrives at the end of the sinter line where the separating blade contacts the unburned fluxed sinter mixture serving as a cleavage layer between the fluxed and unfluxed sintered products. As in the previous embodiment of the invention, all or part of the acid sinter product is preferably recycled and used in the preparation'of the fluxed sinter feed mixture. I

The latter staggered double-layer sinter bed method can be further modified so as to eliminate the necessity of leaving a zone of unburned fluxed sinter mixture between the acid and fluxed sinter layers at the end of the sinter line. Thus, if desired, the upper portion of the acid sinter layer, after burning and before depositing thereon the fluxed sinter mixture, can be contacted with a suitable breaking roll which fractures the upper portion of the acid' sinterlayer to provide a layer of sinter pellets or fragments which can be readily penetrated by the separating blade at the end of the'sinter line, whereby the fiuxed sinter layer can be easily separated from the lower acid sinter layer.

While various forms of apparatus can be employed in carrying out the several embodiments of the present invention, there are illustrated in the accompanying drawings several forms of suitable apparatus adapted to practice the present invention, wherein:

FIGURE 1 is a schematic side elevational view of apparatus suitable for the practice of one embodiment of the present invention;

FIG. 2 is a schematic side elevational view of another apparatus for the practice of a modified form of the present invention; and

FIG. 3 is a fragmentary schematic view of still another apparatus adapted to the practice of a further modified form of the present invention.

In FIGURE 1 of the drawing is shown an iron ore sintering machine of the Dwight-Lloyd type having an endless moving grate 11 with a perforated supporting surface adapted for moving continuously between spaced driving wheels 12, 13 mounted at the opposite ends, respectively, of the said sintering machine 10. The grate 11 travels between spaced side wall sections which extend the length of the sintering machine and provide lateral support for the sinter feed mixture deposited on the moving grate 11. Positioned immediately below the grate 11 are a plurality of contiguous wind boxes 16, 16a which are connected with suitable suction pump means (not shown) by conduits 17, 18, respectively, in order to provide a suitable draft through the sinter bed. The

conduit i8 is connected only to one or more wind boxes designated 16a disposed opposite the point at which the sinter bed burns through the lower layer and immediately adjacent thereto so that gases collected in the said wind box can be conveyed to special treating apparatus. A conventional ignition hood 1% is positioned over the wind box nearest the front end of the sintering machine it).

At the forward end of the sintering machine 10, a sinter feed hopper 20 is positioned above the grate 11 adjacent the front end of the sintering machine. The hopper 26 has an adjustable discharge opening 21 positioned with the lower edge thereof disposed adjacent the surface of the grate 11 and the upper edge a few inches above the surface of the grate 11. The hopper 20 is adapted to deposit a flux-free or acid sinter layer 23, preferably having a depth of between about 3 and 6 inches, directly on the grate 11 or, if desired, on a thin porous hearth layer of inert or sinter material which protects the grate 11 against overheating. Spaced a short distance longitudinally from the forward end of the sintering machine it and hopper 20 is a second hopper 25 having a discharge opening 26 disposed with the lower edge adjacent the upper surface of layer 23 and the upper edge thereof a few inches above the surface of layer 23, whereby a thin layer 27 of material, such as crushed limestone, is deposited on the surface of layer 23. A third hopper 30 is spaced longitudinally from hopper 25 with the lower edge 31 thereof adjacent the upper surface of layer 27 and the upper edge 32 disposed adjacent the top of the side walls of the sintering machine 19. The hopper 3th is adapted to deposit on the surface of layer 27 a relatively thick layer 35 of a fluxed iron ore sinter mixture. The iron ore used in the preparation of the foregoing sinter layers are preferably screened on a inch screen and only the minus 4 inch fraction is used in the preparation of the sinter mixtures. The limestone material used has a particle size less than /8 inch.

The grate 11 having the multi-layer sinter bed formed thereon conveys the three layers of material below the ignition hood 19 where the upper surface of layer 35 containing a predetermined proportion of carbonaceous fuel is ignited and burns from the upper surface downwardly while a draft is drawn downwardly through the bed by the Wind boxes 16, 16a. As the lower layer 23 of the sinter bed burns, the arsenic impurity in the iron ore is released in a volatile form on heating and drawn downwardly through the bed and remains just below the ignition front during the sintering process. The rate of travel of the sinter grate 11 is so adjusted that when the bed reaches the end of the sinter line, the lower sinter layer 23 just burns completely through. Adjacent the section of the sinter bed where the layer 23 burns completely through, substantially all of the arsenic contained in the sinter mixture leaves the bed in the gaseous atmos phere and enters the wind box 16a which is connected with a special conduit 18. The gases from wind boxes 16a are conveyed to special arsenic processing apparatus wherein the arsenic is removed therefrom before discharging the gases to the atmosphere. The finished sinter bed is comprised of a relatively hard fiuxed upper sinter layer 35, an intermediate unsintered layer 2'7 of calcined limestone, and a lower, relatively frangible, sinter layer 23 of an acid or flux-free sinter which has a substantially reduced arsenic content.

At the end of the sinter line, the multi-layer sinter bed engages a substantially horizontally extending separating blade 46 which enters the sinter bed just below the lower surface of layer 35 at the unsintered layer 27, whereby the upper fiuxed sinter layer 35 is readily separated from the lower acid or non-fluxed sinter layer 23. The fiuxed sinter layer 35 is guided into a suitable storage bin 4-1 and the lower acid sinter layer 23 along with the limestone layer 27 is collected in a second bin 42 where the sinter is readily broken into small fragments and all or part thereof conveyed through conduit 43 to the hopper 35 for use in the preparation of the fiuxed sinter mixture.

EXAMPLEl In a specific application of the apparatus of FIGURE. 1

in the process of simultaneously producing arsenic-free acid sinter and a fiuxed sinter, three distinct layers having the composition specified in the following Table I are deposited on the moving grate of the said sintering apparatus and sintered as above described. The acid sinter product and the calcined limestone produced are recycled and used in particulate form as components of the fluxed sinter mixture containingsufficient lime to produce a completely self-fiuxing sinter.

Table I SINTER FEED COMPOSITION AND ARSENIC ANALYSIS [Percent by weight-Fuel free basis] It will be apparent from the foregoing data, that the arsenic content of the fully fluxed or self-fluxing sinter produced in the upper layer is relatively low and that the process has reduced very substantially, the arsenic content of the sinter products. It will be further evident that most of the arsenic removal is effected in the bottom layer of the sinter bed wherein the acid sinter operation takes place.

Studies of the percentage of arsenic at various parts of the acid sinter layer during the sintering operation have shown that the temperature of the sinter layer must reach almost the temperature of fusion before any reduction in arsenic content takes place. Thus, if insufficient fuel, such as coke breeze, is mixed with the iron ore to elfect substantialfusion, there is no significant removal of ar- 3 sonic. For example, in one instance, a sinter bed containing from 2% to'3% coke breeze, there was no significant arsenic removed. When about 3.0% to 4.5% coke breeze is used, up to 78% of the arsenic in the sinter mixture is removed during the acid sintering process. At a coke breeze level of about 5.5%, about 90% of the arsenic present can be removed in the instant sinter process.

It has also been observed that during the sintering of the iron ore according to the present invention, the arsenic is removed from the upper portion of the acid sinter layer and is retained in a wet zone immediatel in front of the gradually progressing fusion zone. Thus, the arsenic removed from the upper portions of an acid sinter layer is first absorbed on the lowerportions of the layer of iron ore sinter mixture before it leaves the sinter bed. Thehot waste gases passing through the sinter bed do not remove the arsenic as long as the latent heat thereof can be absorbed by the lower portions of the acid sinter layer and the temperature thereof remains below about 400 F. The arsenic leaves the acid sinter layer only when the fusion zone progresses to the lower level of the sinter bed.

These findings have resulted in substantial savings being made in the manner of processing the gases released by the sinter bed. Thus, it has been found that only a limited volume of the total gases released need be treated in specialarsenic removing apparatus'in order to effectively recover the objectionable arsenic in the gases and prevent pollution of the atmosphere. By specially treating only a relatively small volume of the gasespassing through the sinter bed, a more complete removalof arsenic is possible. As shown in FIGS. 1 and 2 or" the drawing, the wind boxes disposed below the section of the sintering machine where the lower layer of the sinter bed burns through and immediately adjacent thereto, are connected with a separate draft conduit so that only the gases of these particular areas are conveyed to a special arsenic removal apparatus. More positive control over the pre cise sectors or wind boxes in which the arsenic is removed from the bed can be achieved by controlling the tempera- .ture of the air drawn through the sinter bed by the wind boxes. Thus, for example,it is possible to accelerate the removal of arsenic at one or more critical locations by utilizing preheated air at specific sectors of the sintering machine. Similarly, cooler air can be used at other'sectors of the sintering machine to retard arsenic removal, if this is desired.

EXAMPLE 2 Upper Intermedi- Bottom layer ate layer layer (6 inches) (2 inches) (3 inches) Leslie ore (natural) 42 Marmion ore (natural) 42 100 Return acid sinter of Marmion 0re 0 0 Limestone p 16 0 Coke breeze (fuel) 6 3.

Arsenic:

Before sintering- 0. 022 0. 034 After sintering 0.018 0.009

It will be evident from the data of Table II, that the reduction in the arsenic content of the fluxed sinter product of the upper layer is not as great as the reduction obtained in the lower layer. The fluxed sinter product obtained, however, is found to have'improved strength and good physical properties. The time-temperature curves obtained for this multiple layer sinter bed also indicates that the rate of sintering is greatly increased as compared with that attained in the conventional sintering of a mixture having asimilar composition.

in PEG. 2 of the drawing is shown a modified form of a sintering machine 5% for carrying out the embodiment of the-invention in which two ignition hoods are employed at spaced points along the sintering machine. As in the previous sinter machine 1%, an endless conveyor grate 151 having a perforated lower supporting surface is moved between spaced drive wheels 52, 53 disposed at opposite ends of the sinter machine 5%. The grate 51 is adapted to move between spaced side wall sections which extend the length of'the sinter machine and provide lateral support for a sinter bed disposed on the grate 51. Positioned immediately below the grate 51 are a plurality of contiguous wind boxes 54, 54a which are connected with suitable suction pumps (not shown) by conduits 56, 57, respectively, to provide the required draft through the sinter bed on grate 51. Conduit 57 is connected with the wind box 5 in disposed adjacent the point at which the lower sinter layer burns through in order to convey the arsenic contained gases therefrom to special arsenic removal apparatus.

At the forward endof the sinter machine 5%, a sinter feed hopper 58 is positioned above the sinter grate 51 with the discharge outlet thereof adjacent the surface of the grate ST. The hopper 58 is adapted to deposit an acid sinter mixture layer 59 having a high arsenic content, onto the sinter grate 51. Adjacent the hopper 58 and immediately above the layer 59 is an ignition hood so which ignites the fuelin the acid sinter mixture as a downwardly flowing draft is maintained through the layer of acid sinter mixture by the several wind boxes, 54,54a disposed below the grate 5T. Spaced longitudinally from the hop er 58 and ignition hood 6b are an additional hopper and an ignition hood as. The hopper 65 is positioned at a point intermediate the ends of the sintering machine where the lower sinter layer 5@ has substantially burned through, or at least at a point where the surface zone thereof has completely burned and cooled below the ignition temperature of the upper layer of sinter mixture. The discharge outlet 67 of the hopper 65 is positioned with its lower edge at the upper surface of the layer 59 and the upper edge several inches above the layer 59 so that a fluxed sinter mixture layer 65 having a depth of, about 6 inches is deposited over the layer 59. The outlet 67 is preferably adjustable so that the depth of the layer 6d can be varied. The upper surface of layer 56 is ignited by hood 68 disposed adjacent to the hopper 65. The rate of travel of grate 51 is correlated with the depth of layer as and the draft therethrough so that the upper layer is not completely burned through when the sinter bed reaches the end of the sinter line and presents to the separating blades 7% a thin strip of unburned sinter feed mixture,

as indicated at "71. The separating blade 70 readily enters the sinter bed at the section '71 of unburned sinter mixture, and diverts the upper fluxed sinter product into bin 8d and the lower acid sinter product into bin 81. As in the previous embodiment, the arsenic-free acid sinter product of the lower layer of the sinter bed is easily broken into pellet-like particles and can be conveyed through conduit means 87 to hopper 65 for use in the preparation of the fluxed sinter feed mixture. 7

A modification of the latter apparatus and method is shown in FIG. 3 of the drawing, wherein a sintering machine 949 which is in most respects the same as sin-tering machine 5% shown in FIG. 2 of the'drawing, with the addition thereto, however, of a breaker roll or scarifier extending transversely of the sintering machine 99 between the spaced lateral walls thereof and adjacent the front edge of the hooper 65. The scarifier @5 is provided with a plurality of arms 94 of varying length, which are adapted to contact the upper zone of the lower sinter layer 91 and break the upper portion thereof into small pellet-like fragments which serve as a parting layer 92 to facilitate separating the subsequently formed fiuxed sinter layer 93 from the lower acid sinter layer 91 without having to leave the lower portion of the fluxed sinter layer 93 unburned at the end of the sinter line; thereby increasing the overall efficiency and output of the sinter line.

, It is also possible to deposit a layer of. fiuxing agent, such as limestone, over the surface of the lower sinter layer after the ignition and sintering of at least the upper surface portion thereof to provide a partition layer between the lower sinter layer and the upper sinter layer without leaving a portion of the upper layer unsintered or breaking the upper surface of the lower layer into small fragments.

While it is preferred to form the high-arsenic content layer of sinter mixture as the lower layer in a multilayer sinter bed process of the present invention, it is possible to reverse the sequence of the layers without interfering with the multiple functioning of the process as, for example, in the embodiment of the invention which employs the single-stage ignition of the sinter bed. If the layers are reversed from that specifically described in connection with FIGS. 1 and 2 of the drawing, however, it would also be preferable to use an upward draft sin-tering process, particularly in the double-stage ignition embodiment of the invention, since the upward draft sintering would avoid any tendency for the arsenic in the upper layer to condense on the relatively cooler, previously sintered, lower layer.

It will also beunderstood that the relative proportions of the ingredients of each of the sinter layer mixtures can be varied as required by the particular composition of the iron ores being sintered and operating characteristics of the sintering machine. Thus, the relative height of each of the layers of sinter mixture and the height of the layer of fluxing agent, if employed, can be variedas required. It will also be apparent that the herein disclosed multi-layer sintering process is capable of producing increased amounts of sintered product because of the greater permeability of each of the sinter beds. Thus, for example, in the form of the invention shown in FIG. 1 of the drawing, the permeability of the upper sinter layer is markedly increased by including therein substantial proportions of the sintered product of the lower layer. In the embodiment of the invention shown in FIGS. 2 and 3, the permeability of the sinter bed is further improved by having the lower layer partially sintered or substantially sintered prior to sintering the upper layer of sinter mixture.

While the process of the present invention in its preferred form has been applied to an iron ore having a high-arsenic content of the Marmion type, the present invention is also applicable to other high-arsenic containing iron ores, such as the iron ores from the Steep Rock Region of Canada, and other iron-bearing ores, such as hematite, magnetite and limonite ores, which may have a high-arsenic content.

Although a single specific embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made. The specific description is not intended as a limitation of the scope of the invention, which is defined by the appended claims.

I claim:

1. A process of simultaneously preparing a fiuxed iron ore sinter product substantially free of arsenic and treating an iron ore containing arsenic as an impurity which comprises; continuously depositing on a continuously moving gas-permeable sintering conveyor means a first layer of a substantially flux-free iron ore sinter mixture containing iron ore having at least about .()3% by weight arsenic as an impurity and depositing thereabove a distinct second layer of a fluxed iron ore sinter mixture in which the iron ore has an arsenic content below about .03% by weight, subjecting both said layers of sinter mixtures during movement of said conveyor means to a downdraft sintering heat treatment to effect formation of dis tinct layers of sinter products of said first and second layers of sinter mixture and reduction of the arsenic content of the said iron ore sinter product of said first layer substantially below about .03% by weight, and continuously separating on said sintering conveyor means said layers of sinter products along a cleavage plane between said layers; whereby a fiuxed sinter product suitable for charging into a metallurgical furnace is recovered separately from the sintered iron ore of said first layer of iron ore sinter mixture.

2. A process of simultaneously producing a fluxed iron ore sinter product and treating an iron ore containing arsenic as an impurity comprising; continuously depositing on a gas-permeable sintering conveyor means a first layer of a substantially flux-free iron ore sinter mixture containing in iron ore having an arsenic content in excess of about .03% by weight, depositing on said first layer an intermediate layer of a finely divided iron ore fluxing agent which does not fuse during a sintering heat treatment and which defines a cleavage plane, depositing on said intermediate layer a distinct layer of a fiuxed iron ore sinter mixture having less than about .03% by weight arsenic, subjecting both said layers of sinter mixtures to a downdraft sintering heat treatment while on said conveyor means to effect formation of distinct first and second layers of sinter products and reduction of the arsenic content of the sinter product of said first layer, continuously separating on said conveyor means the distinct layers of sinter products along said cleavage plane defined by said intermediate layer, and recovering said first layer of sinter product separately from said second layer of fluxed iron ore sinter product; whereby said sinter product of said first layer can be used for recycling as a constituent of said second fluxed iron ore sinter mixture.

3. A process of producing a fluxed iron ore sinter product substantially free of arsenic from an arseniccontaining iron ore comprising; continuously depositing on a continuously moving gas-permeable sintering conveyor means a first layer of a substantially flux-free iron ore sinter mixture containing an iron ore having an arsenic content of at least about .03% by weight and depositing thereabove a distinct second layer of a fluxed iron ore sinter mixture having an arsenic content below about .03% by weight, subjecting said layers of sinter mixtures to a downdraft sintering heat treatment to elfect formation of distinct layers of sinter products and reduction of the arsenic content of said iron ore sinter product of said first layer of sinter mixture below about .03% by weight arsenic, continuously separating said layers of sinter products along a cleavage plane therebetween while supported on said conveyor means, recovering separately said sinter products, and continuously recycling the sinter product of said first layer as a constituent of said fiuxed iron ore sinter mixture.

4. A process of sintering as in claim 3, wherein the maximum temperature attained in said sintering heat treatment within said lower layer is maintained substantially below the maximum temperature attained in said upper layer, whereby the sinter product of said lower layer is more readily frangible into small fragments which are adapted for recycling in said process of sintering while said upper layer sinter product is more resistant to fragmentation and in a form which is adapted for use in a metallurgical furnace.

5. A process of sintering iron ore comprising; continuously depositing a layer of flux-free iron ore sinter mixture and a distinct layer of a fiuxed iron ore sinter mixture on a continuously moving sintering conveyor means having a gas-permeable lower supporting surface, subjecting each of said layers during movement of said conveyor means to a sintering heat treatment which produces distinct layers of sinter products, and continuously separatingsaid layers of sinter products on said conveyor means along a cleavage plane lying between said layers of sinter mixtures after said sintering heat treatment is complete; whereby distinct flux-free and fluxed sinter products are recovered from said sinter conveyor means.

6. A process of sintering iron ore comprising; continuously depositing on a continuously moving sintering conveyor means having a gas-permeable lower surface a first layer of'iron ore sinter mixture, continuously subjecting said first layer to sintering heat treatment to'transform a substantial portion of said first layer into a first sinter product, subjecting the sintered exposed surface portion of said firstlayer to a fragmenting force to form a cleavage layer of fragmented sintered product along the surface thereof, and thereafter continuously depositing on said cleavage layer a second layer of a fiuxed iron ore sintered mixture, subjecting said second layer to sintering heat treatment to form a' fiuxed sinter product, continuously separating said first layer of'sinter product from said fiuxed sinter product along said cleavage layer, and recovering said first sinter product separately from said fiuxed sinter product.

7. A process of sintering iron ore comprising; continuously depositing on a gas-permeable sintering conveyor means a first layer of substantially unfiuxed iron ore sinter mixture, depositing on said first layer an intermediate layer of a finely divided iron ore fiuxing agent which does not fuse during a sintering heat treatment and which defines a cleavage plane, depositing on said r a 12 in'termediatelayer a second layer of fiuxed iron ore sinter mixture, subjecting both said layers of sinter mixture to a sintering heat treatment while on said conveyor means to form distinct first and second layers of sinter products separated by said layer of fiuxing agent, continuously separating said layers of sinter products on said conveyor means along said cleavage plane after said sintering heat treatment is complete, and recovering said first layer of sinter product and said intermediate layer separately from said second layer of sinter product; whereby said first layer of sinter product and said intermediate layer of fiuxiug agent are adapted for recycling as a constituent of said flexed iron ore sinter mixture.

References Qited in the file of this patent UNITED STATES PATENTS 1,221,962 Bittmann Apr. 10, 1917 1,280,221 Heilman Oct. 1, 1918 1,508,101 Holmberg Sept. 9, 1924 2,717,205 Edwards Sept. 6, 1955 2,914,395 Davis Nov. 24, 1959 2,941,881 Rausch et :al June 21, 1960 OTHER REFERENCES Poast: Proc. All /LE. Blast Furnace, Coke Oven and Raw Materials, vol. 11, 1952, pages 101106. 

1. A PROCESS OF SIMULTANEOUSLY PREPARING A FLUXED IRON ORE SINTER PRODUCT SUBSTANTIALLY FREE OF ARSENIC AND TREATING AN IRON ORE CONTAINING ARSENIC AS AN IMPURITY WHICH COMPRISES; CONTINUOUSLY DEPOSITING ON A CONTINUOUSLY MOVING GAS-PERMEABLE SINTERING CONVEYOR MEANS A FIRST LAYER OF A SUBSTANTIALLY FLUX-FREE IRON ORE SINTER MIXTURE CONTAINING IRON ORE HAVING AT LEAST ABOUT .03% BY WEIGHT ARSENIC AS AN IMPURITY AND DEPOSITING THEREABOVE A DISTINCT SECOND LAYER OF A FLUXED IRON ORE SINTER MIXTURE IN WHICH THE IRON ORE HAS AN ARSENIC CONTENT BELOW ABOUT .03% BY WEIGHT, SUBJECTING BOTH SAID LAYERS OF SINTER MIXTURES DURING MOVEMENT OF SAID CONVEYOR MEANS TO A DOWNDRAFT SINTERING HEAT TREATMENT TO EFFECT FORMATION OF DIS- 